This invention relates to a voltage controlled oscillator, and, more particularly, to a voltage controlled oscillator which can oscillate high frequency signals with high accuracy, linearity and stability, and a phase lock device using such an oscillator.
As one type of voltage controlled oscillator, an emitter coupled multivibrator has been proposed in a entitled book "BIPOLAR AND MOS ANALOG INTEGRATED CIRCUIT DESIGN", by A. B. Grebene published by John Wiley & Sons Inc. 1984, p571-575. This oscillator, when clamped under the operation condition of 2I.sub.o R2Vd (I.sub.o : constant current at a voltage controlled current source, R: resistance of a load, Vd: drop voltage of a clamping diode), provides an oscillation frequency f obtained by the following equation: EQU f=I.sub.o /4C.multidot.Vd (1)
where C is a timing capacitance.
(1) Voltage controlled oscillators (hereinafter referred to as VCO) are generally desired to meet the requirements of (a) being capable of oscillating with a wide frequency range, (b) low power consumption or dissipation and (c) stability irrespective of various disturbance.
In order to oscillate higher frequency signals, the value of C should be smaller with the value of Vd fixed, as understood from Equation (1). To meet the above requirement of low power consumption, however, I.sub.o must be as small as possible so that the value of C must be smaller still.
Meanwhile, the conventional VCO, formed in the form of an IC without building-in a timing capacitor in an IC chip, provided poor noise endurance. To obviate it, it has been proposed by the inventors of the present invention to build-in the timing capacitor in the IC chip. Thus, since the timing capacitance C is fixed, the oscillation frequency can be controlled by varying the current I.sub.o.
Nevertheless, the oscillation of VCO is not stabilized since the loop gain Gl of this VCO, represented by R/2re=RqI.sub.o /2kT (T: absolute temperature, k: Boltzmann constant, re: emitter resistance) also changes with the current I.sub.o (q/kT is constant with T fixed).
Thus, such a VCO meeting the requirements of (a)-(c) and having a fixed loop gain has been desired to be developed.
(2) Further, the conventional VCO encounters problems that the oscillation frequency f varies or fluctuates due to the fact that Vd varies with T, and that it cannot provide stabilized oscillation operation when the power supply voltage is low, e.g., less than 5 V.
(3) a VCO has been desired to be able to set a stabilized center frequency of the oscillation.
(4) The conventional phase-locked loop (hereinafter referred to as PLL) using a VCO has the following problems. The conventional PLL adopts, as means for holding the frequency in the VCO when input signals disappear, means of switching the VCO input to a sample-and-hold circuit with a large time constant and replacing the input signal by the output signal from a quartz oscillator, as disclosed in e.g. JP-A-No. 57-160244. However, when the sample-and-hold circuit has a large time constant, the rising of the output frequency in the VCO, after the input signal has reappeared, is retarded so that the output frequency in the VCO is stabilized late. The frequency holding means is so complicated and expensive that it is not suitably implemented in the form of a monolithic integrated circuit.